While it is possible to run 200 simultaneous calls on one server I would highly suggest you break it up to 2 or more servers. Most large installations shoot for one 4 port card (about 100 calls) per server. Also because the telephony cards use so many interrupts it is also not recommended to run more than one telephony card per server anyway.
Any new single physical processor server with 2-4 GB RAM will work just fine for 100 calls unless you are doing a lot of transcoding and\or call recording. If you do either or both then go for the second physical cpu and go with 4GB RAM. If recording make sure you have a decent disk subsystem (15K RPM SAS drives recomended).